Many photographic materials contain gelatin as the major component in most of their constituent layers, e.g., a silver halide light-sensitive emulsion layer, an emulsion protecting layer, a filter layer, an interlayer, an antihalation layer, a backing layer, a film base subbing layer, a baryta layer.
Such gelatin-containing light-sensitive materials are treated with aqueous solutions differing in pH or temperature, causing layers containing unhardened gelatin to swell excessively. Such swollen gelatin-containing materials scratch easily due to their tendency to absorb water and soften. In extreme cases, gelatin-containing layers are dissolved in high-temperature processing solutions, particularly at 30.degree. C. or above, and are transferred from the light-sensitive materials into the processing solutions, which is highly undesirable.
A large number of hardening compounds are known to be effective in enhancing the water resistance, heat resistance and scratch resistance of gelatin layers. These compounds are well known as "hardeners" employed in the production of photographic materials, and include, for example, inorganic compounds like chrome alum, and organic compounds including aldehydes such as formaldehyde and glutaraldehyde, active halogen-containing compounds described in U.S. Pat. No. 3,288,775, compounds having reactive ethylenically unsaturated groups described in U.S. Pat. No. 3,635,718, aziridine series compounds decribed in U.S. Pat. No. 3,017,280, epoxy compounds described in U.S. Pat. No. 3,091,537 and halogenocarboxyaldehydes such as mucochloric acid.
However, photographic materials are also required to have heightened covering power (optical density obtained by a definite amount of coated silver). In order to increase covering power it is desirable to increase the swelling degree of silver halide emulsion layers at the time of development by using gelatin which is hardened to a lessened degree. By this means silver is saved and the required optical density is achieved using a minimum amount of coated silver.
The effect of enhancing covering power by decreasing the degree of hardening is notable particularly in high temperature processing. However, decreasing the degree of hardening as described above results in excessive swelling of the silver halide emulsion layers upon high temperature processing and consequently, the silver halide emulsion layers are likely to be scratched, or in extreme cases to be dissolved and removed. Therefore, the degree of hardening of a photographic material which is to be wet processed under high-temperature conditions is determined by balancing covering power ("degree of swelling") and scratch resistance ("scratching resistance").
If the thickness of the film when dried ("dry thickness") and its thickness when swollen ("wet thickness") are represented by d and d+.DELTA.d, respectively, then the "degree of swelling", which relates to covering power, can be determined by dividing the difference between wet thickness and dry thickness, .DELTA.d, by the dry thickness, d, to obtain .DELTA.d/d.
If a coated film in a swollen condition is scratched with a needle-like point as the load applied to the point is continuously increased, then "scratching resistance" is defined as the load applied to the coated film at its fracture point. The thus-defined "scratching resistance" closely describes the scratch resistance exhibited by a photographic material in wet development, fixation and washing processing. When this value is small, a film is scratched by contacting another film during development processing, and when using an automatic developing machine, delamination of a coated layer may be caused by contact with the carrying rollers.
In photographic materials having silver halide emulsion layers in which conventional silver halide grains are used, i.e., spherical grains, or polyhedral or twin grains having a spherical-like shape, the two contradictory requirements of a proper degree of swelling and a sufficient scratching resistance, can be satisfied by using conventional low molecular weight hardeners described above. However, in photographic materials having silver halide emulsion layers containing tabular silver halide grains with a large diameter to thickness ratio, a swelling degree equivalent to that of spherical grain-containing photographic materials results in a marked decrease of scratching resistance. Accordingly, when a photographic material has at least one silver halide emulsion layer containing tabular silver halide grains, its hardening degree must be increased to achieve a very low swelling degree and to improve its scratch resistance in a wet condition. As the result, the covering power is necessarily reduced.
A method of improving scratch resistance by using a particular hardener, such as a vinylsulfonyl group-containing ether, formaldehyde, or mucochloric acid in a tabular grain emulsion to achieve a greatly diminished swelling degree is described in Japanese Patent Application (OPI) No. 111933/83 (the term "OPT" as used herein refers to a "published unexamined Japanese patent application") (corresponding to U.S. Pat. No. 4,414,304).
However, this method has the disadvantage that the high covering power and high sensitivity inherent in a tabular grain emulsion are deteriorated by greatly diminishing the swelling degree.